RCABS Watertight Container Problems

[mcaswell]

From an email from John Slater in Australia

Re the ballast systems I have used. There is nothing fundamental wrong with it initially, but as frequent sub driver the longevity of the system is questionable. Further the actual Water tight integrity I have found has also diminished overtime with the WTC compartments I have employed despite regular care and maintenance.

The system I used pumped water into a sealed ballast tank via a check valve. A servo runs a micro switch which activates the pump, which pumps in water to the tank through a one way valve tank. The air inside the tank has no where to go so its actually wanting to force the water back out but the valve keeps the water in the tank. The boat dives with the water in the tank. To surface, the servo (which activates the pump) now opens the valve. You can do the same with a bladder in place of the solid tank, and both systems do work very well (up to a point).

Problems however are these:

Overtime, if your tank is a Bag, these do break via small cracks - if you dived your stuffed! Bags also don't inflate so well against water pressure at depth.

Pumps frequently need replacing.

The construction of a rigid tank ( my main system I used) to hold the pressure is ok, but pump selection is crucial. An impeller pump will safely keep turning but not keep pushing water in to the tank at around 10psi. A geared pump is like a time bomb, if somehow the servo to the microswitch jams, the boat will go off like a depth charge. Have seen this happen- wish I had a camera or video!

If you employ an impeller pump -and for safety its a must, it will only fill the tank to 60% capacity (40% of the tanks volume holds compressed air), this means your ballast tank has to be 40% bigger than what it should be in an equivalent gas boat. Related to this issue is that to get any tank 60% full using an impeller pump requires 12 volts of power. If you wanted to have your WTC setup using say an RC car battery (7.2 volts) your tank would only fill to around 35%, meaning your ballast tank would have to be 65% bigger than required. Therefore trimming the boat for proper surfaced and submerged trim is more problematic too! In a bigger boat its not that bad running at 12 volts, but in smaller boats this system is a pain to setup right.

Weird electrical glitches happen too, as after a while, even ball raced indirect drive (high torque) servos that work against the clippard release valve seem to burn out and the end result is the servo malfunctions which usually then makes the receiver go haywire.

Basically - it you want to replace component after component then you could live with my old system, but I have hit a point of wanting the best, with minimum of fuss. This is where David's system comes into play. Sure you have consumables (gas), but his gas system with the on board gas saver is like no other gas system. The reason why I tried gas initially and went in the direction of pumps was that the gas system I used, wasted a lot of gas on unecessary blows. With David's system when the water is out of the tank - you don't waste gas, its like having your own personel on board the boat monitoring what state the ballast tank is in. Sure you have to be careful in how many vent blow cycles you go through but that is easy, plus as the gas tank emptys the trim will be higher.

IMO - the new RCABS system and RCABS reverse system to is also a problem. I have built both and had failures. With RCABS (air filled bladder to surface) the system is flawed because you need power other than a servo movement, to inflate a bag. Surfacing IMO should require minimal electrical effort such as a simply servo movement. Also at depth compression on the bag is more crucial than a rigid WTC, so the water pressure is actually wanting to deflate the air bag. I have had my Alfa operating at depths down as far as 5 metres in an Olympic dive pool, and the RCABS was useless past 2 metres down and struggled at around 1.5 metres. (My old pump system - pumping into a sealed tank worked a treat at 5 metres depth.That's 16.4 feet and IMO beyond all operational norms!) RCABS reverse whilst having minimal electrcial effort to surface has depth problems too via the bag is inflating again against water pressure. Further it has complicated plumbing arrangements. Now you are probably not wanting to go that deep, most of the time in lakes I go no deeper than a couple of feet, but nontheless its a limitation on the bags systems. You only need a slight drop in power from a battery going through the norms of being used and bag inflation boats run into trouble. RCABS and R-RCABS typically run the whole boat on 6 volts, but as this voltage is consumed RCABS in particular struggles further to reinflate the bag at depths of around 4-5 feet.

John Slater

[Merriman]

Simple is good.

David D Merriman lll
D&E Miniatures